Process of recovering metals from zinc plant residue



y 1970 a. ORLANDINI ETAL 3,510,414

PROCESS OF RECOVERING METALS FROM ZINC PLANT RESIDUE Filed Dec. 4'. 1967I I l TREATMENT i I H so I H I r f I STRONG new i g I uncn l :5. (DRonsm/c I i I V nzswua;

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INVENTORS.

Bnurvo Onlnndm: Hagen Sc/vr-n Hnofh flffy j United States Patent O3,510,414 PROCESS OF RECOVERING METALS FROM ZINC PLANT RESIDUE BrunoOrlandini and Walter Schmittroth, Kellogg, Idaho,

assignors to The Bunker Hill Company, a corporation of Delaware FiledDec. 4, 1967, Ser. No. 687,524 Int. Cl. C22d 1/22; (32% 19/02; C01g 9/06US. Cl. 204-119 Claims ABSTRACT OF THE DISCLOSURE A three stage processis described for recovering metals particularly zinc, from zinc plantresidues. The zinc plant residue, in first stage, is first leached instrong sulphuric acid solution for 6-8 hours at a temperature between 90C. and 94 C. to dissolve the zinc ferrites and other zinc compounds, andthen settled or filtered to produce a first stage residue and solution.The first stage residue may be returned to a smelter for recovery ofcontained metal values. In the second stage, the clarified solution fromthe first stage is neutralized by a suitable neutralizing agent such aszinc oxide to reduce the acid concentration to a pH level of 5.0-5.3 toprecipitate iron compounds and other impurities. After settling orfiltering, a second stage residue and clarified solution are produced.The clarified solution is returned to the normal zinc plant leach tankfor recovery of zinc values. The second stage residue is then dried byheating the residue to a temperature between 110 C. and 140 C. In thethird stage, the dried residue is leached in a weak sulphuric acidsolution to dissolve the contained zinc. The resulting leach pulp isfiltered to produce a final low zinc containing residue, and a thirdstage filtrate which is returned to the normal zinc plant leach circuit.

BACKGROUND OF THE INVENTION This invention relates to hydrometallurgicalextraction of metals and more particularly it relates to the recovery ofthe zinc and other metals from zinc plant residues.

In the conventional process for recovering zinc from the zinc sulfideores, the concentrated ore is roasted in an oxidizing atmosphere totransform a considerable portion of the zinc into a more acid solublecompound of Zinc oxide. During the roasting process some of the zinccombines with the iron to form an insoluble zinc ferrite, generallybelieved to have a formula ZnFe O The roasted ore is frequently referredto as zinc calcine. The calcine is then leached in a weak sulfuric acidsolution to dissolve the zinc oxide to form zinc sulfate solution. Theinsolubles are separated from the solution by a filtration system. Thecalcine leach residue contains a high percentage of the zinc ferrite andlesser amounts of other insoluble materials such as zinc sulfide, lead,gold and silver. The calcine leach residue is generally referred to aszinc plant residue.

The zinc sulphate filtrate is then purified to remove many of thesoluble impurities before the filtrate is utilized as the electrolyte inthe electrolytic process for producing cathode zinc. The sulfuric acidsolution generated during the electrolytic deposition of zinc, referredto as spent electrolyte, is frequently utilized as the agent forleaching the calcine.

Commonly, the zinc plants residue contains between -30% of the zinc.This represents a substantial loss. Considerable research has beenconducted in the past 30 years in an effort to develop commerciallyfeasible processes for recovering zinc from such residues.

The principal purpose of this invention is to provide ice a commerciallyfeasible process for recovering zinc and other metals from zinc plantresidues.

An additional object of this invention is to provide a process forrecovering zinc from zinc plant residues that is efficient, simple andrequires less capital outlay than those processes that are presentlyused.

These and other objects and advantages of this in- 'vention will becomeapparent upon a reading of the following detailed description of apreferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of thisinvention is illustrated in the accompanying drawing showing a schematicflow diagram of the principal steps of this process invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The zinc is frequentlymined from the earth in the form of a zinc sulfide ore. The zinc sulfideore contains an appreciable amount of iron compounds and lesser amountsof other metallic materials such as copper, silver, gold, lead,antimony, cobalt and cadmium. After the mining operation the ore isconcentrated by flotation to increase the concentration of zinc sulfide.The concentrated ore is then roasted to convert the zinc sulfide into amore soluble compound of zinc oxide. During the roasting process anappreciable amount of zinc combines with the iron to form a zincferrite. Roasted ore concentrates are generally referred to as calcine.

The calcine is then leached in a weak sulfuric acid solution to dissolvethe zinc oxide to form a zinc sulfate solution. Some of the othermetallic compounds also dissolve. Many of the other compounds areinsoluble in the weak sulfuric acid solution and fail to dissolve.Included in the insoluble compounds are zinc ferrite and a lesser amountof zinc sulfide that Was not completely oxidized during the roastingprocess. Gold, silver and lead are included in the insolubles. Theresidue frequently contains about 30% of the cadmium and about 50% ofthe copper contained in the ore. After the leaching process, thesolution flows through a filtration system to separate the solution fromthe residue.

The principal portion of this invention concerns the recovery of themetals, particularly zinc, cadmium, copper from the residue that isfiltered from the calcine leach pulp. This residue as previouslymentioned, is referred to in the industry as zinc plant residue. Thezinc plant residue is processed in a three stage process in which theresidue is first leached in a strong sulfuric acid solution having asulfuric acid concentration in excess of the stoichiometrical equivalentto the zinc in the residue. It has been found that 20 to 50 grams perliter of excess sulfuric acid in the leach solution is advantageous. Thestrong sulfuric acid leach solution is maintained at a temperaturebetween -94 degrees C. to increase the dissolution of the zinccompounds. The copper, cadmium and iron are also dissolved to formsoluble sulfates. The lead, silver and gold do not dissolve. It is foundthat it is advisable to leach the residue for six to eight hours. Theleach pulp is then processed through a filtration or settling system orboth in which the insoluble materials (lead, silver and gold) areremoved from the sulfate filtrate. The residue from this step isgenerally further processed in a smelter to recover the lead, silver andgold. From the filtration step the sulfate filtrate is processed throughthe second stage in which the filtrate is neutralized to a pH of between5.0 and 5.3 by the addition of zinc oxide or other suitable neutralizingagent such as cadmium oxide to precipitate the iron sulfate [Fe (SO andferric hydroxide [Fe(OH) Manganese dioxide is also added to the filtrateto oxidize the ferrous iron to the ferric state to insure completeprecipitation of the iron.

The neutralized solution is maintained at a temperature 3f about 90 C.for several hours. The neutralizing step is completed when the totalsoluble iron concentration in :he solution is less than 0.5 gram perliter. The solution Is neutral to methyl orange. The neutralizedsolution is :hen passed through a filtration or settling system or both,to separate the residue from the solution. The filtrate, con- :ainingsoluble zinc, cadmium, and copper sulfate, is added to the calcine leachtank as is shown in the ac- :ompanying drawing.

The predominantly iron residue generally contains appreciable amounts ofentrained soluble zinc compounds. The entrained zinc as recovered by thefollowing important third stage steps.

The residue is first dried at temperatures between 110 and 140 C. torecrystallize the zinc to soluble zinc compounds. After the residue hasadequately dried, the residue is ground into small particles and thenleached in a weak sulfuric acid solution of between -8 grams of sulfuricacid per liter of solution to dissolve the soluble zinc compounds, andform a zinc sulfate solution. The solution is then passed through afiltration system to separate the zinc sulfate from the residue. Thefiltrate is added to the calcine leach tank. The resultant zinc sulfatefiltrate is purified to remove and recover the copper and cadmium. Thepurified zinc sulfate filtrate is utilized as the electrolyte in theelectrolytic process for recovering the zinc as cathode zinc.

By utilizing the above process over 80% of the zinc in the zinc plantresidue may be recovered.

Example A 757 gram sample of the zinc plant residue contained 19.9%zinc, 19.4% iron, 7.9% lead and 19.8% of other material includingcopper, cadmium, gold and silver. The residue was leached in a 5 litersulfuric acid solution containing 1.20 grams of sulfuric acid per literof solution. The residue was leached for 6 to 8 hours at approximately90 C. At the end of the period this solution had a concentration ofgrams sulfuric acid. It was found that 84.4% of the copper was dissolvedinto solution and approximately 84% of the iron dissolved. The leachpulp was filtered to separate the uudissolved portions from thesolution. An assay of the remaining residue showed the followingcomposition: 4.4% zinc, 8% iron, 14.5% lead and small amounts of copperand gold.

The filtrate was then neutralized by the addition of suflicient amountsof zinc oxide to bring the solution to a pH value of 5.0 to 5.3 (neutralto methyl orange) to precipitate the iron compounds. This solution ismaintained at a temperature of approximately 90% C. for several hours.Provisions are made for the addition of manganese dioxide or anothersuitable oxidizing agent to oxidize the ferrous iron to the ferricstate. The step is complete when the soluble iron concentration is lessthan 0.5 gram per liter. The solution is then filtered to separate thepredominantly iron sulfate precipitate from the sulfate solution. Anassay of the residue showed that it contained 26.8% zinc. To recover asubstantial portion of the remaining zinc values from the residue theresidue was processed through the third stage of the zinc plant residuetreatment. The residue was heated to a temperature of between 100 and140 C. to recrystallize the contained zinc compounds for facilitatingadditional leaching. After drying the residue, it was ground into fineparticles and then leached in a weak acid solution of between 2 to 3grams of sulfuric acid per liter of solution at a temperature ofapproximately 90 C. for one hour. The leach pulp solution was thenfiltered to remove the residue from the sulfate filtrate. It was foundthat 90.2% of the zinc was recovered from the first stage residue.

It should be understood that the above described embodiment could bealtered and modified without deviating from the principle of thisprocess invention.

What we claim as new and desire to protect by Letters Patent of theUnited States is:

1. A process for the extraction of zinc from zinc plant residuecontaining zinc ferrite comprising the steps of:

(a) leaching the residue with a strong sulfuric acid solution todissolve at least a portion of the zinc ferrite;

(b) neutralizing the solution with a neutralizing agent to precipitateat least a portion of the iron;

(0) filtering the solution to separate the iron precipitateresidue fromthe solution;

(d) drying the iron precipitate residue at a temperature of between 110degrees and 140 degrees C.;

(e) leaching the dried iron precipitate residue with a weak sulfuricacid solution to dissolve any remaining soluble zinc values from theresidue;

(f) filtering the solution to separate the insoluble iron residue fromthe zinc sulfate solution.

2. A process as defined in claim 1 wherein the zinc plant residue isleached in a strong sulfuric acid solution at a temperature of 94degrees C.'

3. A process as defined in claim 2 wherein the strong sulfuric acidsolution has an initial sulfuric acid concentration of 20 to 50 gramsper liter in excess of the stoichiometrical equivalent to the zinc inthe residue.

4. A process as defined in claim 1 wherein the solution is neutralizedto a pH level of 5.0 to 5.3.

5. A process as defined in claim 4 wherein the neutralizing agent iszinc oxide.

6. A process as defined in claim 1 wherein the dry iron precipitateresidue is leached at a temperature of approximately 90 C. by a weaksulfuric acid solution having an initial sulfuric acid concentration ofbetween 0 and 8 grams per liter of solution.

7. In a process for recovering metallic zinc from zinc sulfide orecontaining iron compounds by separating the ore to form a zincconcentrate, roasting the concentrate to form a zinc calcine containingzinc oxide and zinc ferrite, leaching the calcine to dissolve the zincoxide to form a calcine leach pulp, filtering the calcine leach pulp toseparate the zinc ferrite residue from the leach solution, purifying thecalcine leach filtrate to form a zinc sulfate electrolyte and depositingthe zinc on a cathode by electrolytic deposition, in which the processincludes the improvement of a leaching sub-process for recovering thezinc from the zinc ferrite residue by performing the steps of:

(a) leaching the zinc ferrite residue with a strong sulfuric acidsolution to dissolve at least a portion of the zinc ferrite;

(b) neutralizing the solution with a neutralizing agent to precipitatethe iron to form a leach pulp;

(c) filtering the leach pulp to separate the precipitate from thesolution to form a filtrate and a residue;

(d) drying the residue by heating the residue to a temperature ofbetween and C.;

(e) leaching the dried residue in a weak sulfuric acid solution todissolve any soluble zinc values that may be contained in the driedresidue to form a second leach pulp; and

(f) filtering the second leach pulp to separate the insoluble ironresidue from the zinc sulfate solution to form a resultant filtrate.

8. In a process as defined in claim 7 wherein the dried second residueis leached in a weak acid solution having concentration of between 0 and8 grams of sulfuric acid per liter of solution at a temperature ofapproximately 90 C.

9. In a process as defined in claim 8 wherein the strong sulfuric acidleach solution is neutralized by a suflicient amount of zinc oxide toreduce the acid concentration to a pH level of between 5.0 and 5.3.

5 6 10. In a process as defined in claim 7 further includ- 1,761,7826/1930 Fischer 23125 ing the sub-process step of adding the filtrates t0the 1,255,438 2/1918 Laist 23-125 calcine leach prior to thepurification process.

JOHN H. MACK, Primary Examiner References Cited 5 R. L. ANDREWS,Asslstant Examiner UNITED STATES PATENTS 3,143,486 8/1964 Pickering eta1. 2041 19 US. Cl. X.R. 2,772,230 11/1956 Hollander et a1. 204-119 23125; 75 12

